With the recent trend toward size reduction in electronic appliances, there is a desire for a further increase in the capacity of high-capacity secondary batteries. Attention is hence focused on lithium ion secondary batteries, which have a higher energy density than nickel-cadmium and nickel-hydrogen batteries.
Lithium secondary batteries employ a nonaqueous electrolyte obtained by dissolving an electrolyte such as, e.g., LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, LiN(CF3SO2)2, or LiCF3(CF2)3SO3 in a nonaqueous solvent such as a cyclic carbonate, e.g., ethylene carbonate or propylene carbonate, a linear carbonate, e.g., dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate, a cyclic ester, e.g., γ-butyrolactone or γ-valerolactone, a linear ester, e.g., methyl acetate or methyl propionate, a cyclic ether, e.g., tetrahydrofuran, 2-methyltetrahydrofuran, or tetrahydropyran, a linear ether, e.g., dimethoxyethane or dimethoxymethane, or a sulfur-containing organic solvent, e.g., sulfolane or diethyl sulfone.
Secondary batteries employing such nonaqueous electrolytes considerably differ in battery performance because reactivity varies depending on the compositions of the nonaqueous electrolytes. In particular, influences of the decomposition and side reactions of electrolytes on the cycle performance and storage performance of the secondary batteries have become problems. Attempts have hence been made to mitigate these problems by adding various additives to the electrolytes.
For example, patent document 1 describes a technique in which a nonaqueous electrolyte containing at least one additive selected from lithium monofluorophosphate (Li2PO3F) and lithium difluorophosphate (LiPO2F2) is employed. In this technique, the additive is reacted with lithium to form a coating film on the surface of the positive electrode and the negative electrode and thereby inhibit the electrolyte from decomposing due to contact with the positive-electrode active material and the negative-electrode active material. Self-discharge is thus inhibited and storage performance after charge is improved.
Patent document 2 describes a technique in which lithium carbonate is added as an additive to an electrolyte for lithium secondary batteries which has been prepared by dissolving a lithium salt in a nonaqueous solvent including a cyclic ester to thereby improve the charge/discharge characteristics of a battery. There is a statement in this document to the effect that by adding lithium carbonate to the electrolyte beforehand, the lithium carbonate generated by the reaction of the cyclic ester with lithium is prevented from dissolving, whereby lithium is inhibited from reacting with the solvent. Because of this, lithium carbonate is added to the electrolyte preferably in such an amount as to result in a supersaturated state to cause the additive to be present as lithium carbonate in the electrolyte and thereby maintain the effect of the invention.
Patent documents 2 and 3 include a statement to the effect that some kind of difluorophosphate is useful as an additive for an electrolyte for lithium batteries. However, patent document 3 includes a statement to the effect that addition of a salt mixture comprising lithium difluorophosphate and lithium monofluorophosphate results in poorer battery performances than in the case of adding sodium difluorophosphate. As apparent from these, details of the effect of that additive and conditions for the use thereof, e.g., as to what salt of difluorophosphoric acid is suitable, have not been fully elucidated.
Furthermore, non-patent document 1 includes a statement to the effect that when CO2 or Li2CO3 is caused to be present as an additive in an LiPF6 solution, then the lithium cycle efficiency improves and that Li2CO3 is an excellent coating agent.
Although those techniques can mitigate the problems in some degree, they are not always satisfactory. There is a desire for a proposal on a technique which is industrially advantageous and produces effects with higher certainty. In particular, it has been thought that a difluorophosphate can be produced by reacting, e.g., P2O3F4 with a metal salt and NH3 (see non-patent document 2 and non-patent document 3). This technique, however, has been extremely disadvantageous for use as an industrial-scale process for producing a difluorophosphate as an additive for nonaqueous electrolytes, because P2O3F4 as a raw material is difficult to procure and exceedingly expensive and purification by by-product separation is necessary.    Patent Document 1: JP-A-11-67270    Patent Document 2: JP-A-1-286263    Patent Document 3: Japanese Patent No. 3438085    Patent Document 4: JP-A-2004-31079    Non-Patent Document 1: J. Electrochem. Soc., Vol. 143, No. 12, December 1996, pp. 3809-3820    Non-Patent Document 2: J. Fluorine Chem. (1988), 38(3), pp. 297-302    Non-Patent Document 3: Inorganic Chemistry, Vol. 6, No. 10, pp. 1915-1917 (1967)